The invention relates to receiving multi-carrier signals, and, more particularly, to receiving multi-carrier signals with dynamic power adjustment.
Multi-carrier modulation methods, such as orthogonal frequency division multiplexing (OFDM), have been around for some time now. OFDM is a modulation method designed in the 1970's in which multiple symbols are transmitted in parallel using different sub-carriers. An OFDM system forms its symbol by taking k complex QAM symbols Xk, each modulating a sub-carrier with frequency
            f      k        =          k              T        u              ,where Tu is the sub-carrier symbol period. Each OFDM sub-carrier displays a
      sin    ⁢                  ⁢          c      ⁡              (        x        )              =            sin      ⁡              (        x        )              x  spectrum in the frequency domain. By spacing each of the 2N+1 sub-carriers
  1      T    u  apart in the frequency domain, the primary peak of each sub-carrier's sinc (x) spectrum coincides with a null of the spectrum of every other sub-carrier. In this way, although the spectra of the sub-carriers overlap, they remain orthogonal to one another. OFDM is well known as a highly spectral efficient transmission scheme capable of dealing with severe channel impairment encountered in a wireless environment. The basic idea of OFDM is to divide the available spectrum into several sub-channels (sub-carriers). By making all sub-channels narrowband, they experience almost flat fading, which makes equalization very simple.
Mobile reception, however, remains one of the problems associated with OFDM systems. A moving receiver would experience Doppler shifts, which corrupt the orthogonality among each sub-carrier, and decrease the system performance. In such a case, inter-carrier interference (ICI) occurs because signal components from one sub-carrier cause interference with other, mostly neighboring, sub-carriers. Moving receivers also encounter time-varying channels. Time-varying channels also limit the system performance. Typically, an ICI canceller is applied to compensate ICI, and a long interleaver with additional forward error correction would enhance system performance. For example, Digital Video Broadcast Hand-held (DVB-H) has specified the use of multiprotocol encapsulation forward error correction (MPE-FEC) to provide an additional layer of interleaving and error correction in order to provide a more robust signal in mobile environments.
Although ICI cancellation and MPE-FEC can improve system performance, these two features consume relatively more power. Since handheld devices have limited battery power, system designers must compromise between system performance and power consumption.